The present invention relates, in general, to the field of computer bus signal termination systems. More particularly, the present invention relates to an automatic signal termination system for a computer bus of especial utility with daisy chain busses requiring end termination such as the small computer system interface (SCSI).
The SCSI bus is a parallel, multimaster input/output (I/O) bus which has become a de facto standard for interconnection of computers and peripheral devices since its adoption as an ANSI standard in 1986. SCSI (8-bit) can support up to eight devices (host adapters or peripheral controllers) although each device can have eight logical units, with each logical unit having 256 logical subunits. The SCSI bus can be either single ended, wherein each signal's logic level is determined relative to a common ground, or differential wherein the logic level is determined between pairs of conductors. SCSI-2, which evolved from the original SCSI specification, standardized the way bus terminators are powered, makes certain messages and command sets mandatory and provides for two optional enhancements to increase bus throughput, which are: fast SCSI (up to 10,000,000 byte transfers per second) and wide SCSI (16 or 32 bit wide data path). SCSI is, therefore, a robust and popular bus for the interconnection of computer host and peripheral devices.
The SCSI bus must, however, be terminated at both ends (and only at the ends) for proper operation. In fact, the SCSI-2 standard for single ended SCSI recommends active termination at both ends of every cable segment. Should the bus not be terminated properly, the devices coupled to the bus may either cease functioning properly, thereby introducing serious error problems, or cease functioning at all. Conversely, should the SCSI bus be "over" terminated, i.e. terminated at more places than the ends, signal reflections and other signal complications will likewise lead to high failure rates and errors.
As additional host adapters or peripheral controllers are added to an existing SCSI bus (or the buses are reconfigured), care must be exercised in properly configuring the bus terminators. Previously, termination has been effectuated by means of passive single inline package (SIP) resistor packages, loop back connectors or other relatively expensive terminator plugs. Thus, a knowledgeable technician must accurately access the bus configuration and terminate the bus ends properly as well as take care that such removable devices are not misplaced or lost if the configuration is changed.
Recently, active terminator devices have become available to ameliorate problems attendant misplaced or lost terminator plugs, SIP resistor packs or loop back connectors. Such active devices, as for example the UC 5601 small computer system interface active terminator available from Unitrode Integrated Circuits provide active termination for all 18 SCSI signal lines by resistive pull up to a 2.9 volt reference. A disconnect signal will also effectively remove the terminating resistors from the bus. Nevertheless, appropriate enablement or disablement of a given active terminator, as with the passive terminators, still requires a rather sophisticated degree of technical competence, ordinarily beyond the capacity of the average end user. The addition of a peripheral device to a previously configured bus is therefore, still too complex for a customer who must, of necessity, be schooled in what switch to throw or jumper connection to make to configure his system.
Certain SCSI computers from Apple Computers, Inc., for example the McIntosh SE, have nevertheless had power supplies too weak to power a terminator thereby still requiring the addition of one or two external terminators when connecting SCSI devices. The instructions for adding the external terminators fill a few pages of the relevant manuals.
The existing passive and active termination techniques have, therefore, relegated the task of properly terminating the SCSI bus to only those with sufficient training and knowledge. Even for those with the requisite skills the task nevertheless remains time consuming and tedious.